A universal dividing head is a precision machine tool accessory primarily used to accurately divide the circumference of a workpiece into precise, equal angular divisions. While a "dividing head sketch" doesn't have a functional role itself, sketches are invaluable tools for illustrating the complex internal mechanism and operational principles of a dividing head, making its function easier to understand.
Function of a Dividing Head (and why sketches help)
The primary function of a dividing head is to enable accurate indexing or angular positioning of a workpiece. This is crucial for creating features like gear teeth, splines, flutes on reamers, polygons, or bolt circles on cylindrical or flat workpieces.
- Precise Angular Division: Its main purpose is to rotate a workpiece by a specific, controlled angle, allowing for operations like milling multiple flats on a shaft or cutting gear teeth with uniform spacing.
- Helical Milling: A universal dividing head can also coordinate the rotary motion of the workpiece with the linear motion of the milling machine table. This capability is essential for operations such as cutting helical gears, twist drill flutes, or helical reamer flutes.
Sketches, diagrams, or visual aids are fundamental in explaining these functions by:
- Visualizing Components: Clearly showing the arrangement of gears, plates, and shafts.
- Illustrating Motion: Demonstrating how rotation of one part translates to another, particularly the precise indexing movement or the synchronized helical motion.
- Simplifying Complex Mechanisms: Breaking down the intricate internal workings into understandable segments.
Internal Mechanism of a Universal Dividing Head
The universal dividing head's precision lies in its robust internal mechanism, which combines various mechanical principles to achieve accurate angular positioning and synchronized motion.
Core Components and Their Roles:
| Component | Primary Role |
|---|---|
| Worm and Worm Wheel | The heart of the indexing mechanism, typically with a 40:1 reduction ratio. |
| Indexing Plate | Provides holes in specific patterns for precise angular divisions. |
| Crank and Plunger Pin | Used with the indexing plate to manually select and lock angular positions. |
| Change Gears | Enable differential indexing and crucial for helical milling operations. |
| Auxiliary Input | Connects to the milling machine's leadscrew for helical milling. |
| Spindle | Holds the workpiece via chucks, collets, or centers. |
Detailed Mechanism:
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Worm and Worm Wheel System:
- At the core of the dividing head is a precision worm and worm wheel set. Most universal dividing heads use a standard ratio of 40:1. This means that 40 complete revolutions of the worm (controlled by the indexing crank) are required to make the dividing head's spindle (and thus the workpiece) complete one full revolution (360 degrees).
- This high reduction ratio allows for very fine angular adjustments. For example, if 40 turns give 360 degrees, then 1 turn gives 9 degrees. Further divisions are achieved using indexing plates.
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Indexing Plates and Crank:
- The indexing plate is a circular disc attached to the worm shaft, featuring concentric circles of precisely drilled holes. Common plates have hole circles like 49, 47, 43, 41, 39, 37, 33, 31, 29, 27, 23, 21, 19, 17, etc.
- The indexing crank rotates around the center of the plate and has a plunger pin that engages with the holes on the chosen circle. A sector arm (or arms) is used to set the desired number of holes to move for each index.
- Simple Indexing: To index the workpiece, the operator calculates the required number of turns and holes on a specific circle. For example, to divide a workpiece into 40 equal parts, the crank would be turned one full revolution for each part (40/40 = 1). To divide into 20 parts, the crank would be turned two full revolutions (40/20 = 2). For odd divisions (e.g., 7 parts), fractions of a turn are used, such as 40/7 turns, which translates to 5 full turns and 5/7 of a turn (requiring a 7-hole circle).
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Differential Indexing:
- For divisions that cannot be achieved by simple indexing with the available indexing plates, differential indexing is employed. In this method, the indexing plate is not stationary but is connected to the dividing head spindle through a train of change gears.
- As the indexing crank is turned, the change gears cause the indexing plate to rotate either slightly forward or backward. This effectively "changes" the number of holes available for indexing, allowing for a much wider range of divisions.
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Helical Milling Mechanism (Auxiliary Input and Gear Train):
- A unique feature of universal dividing heads is their ability to perform helical milling. This capability is facilitated by an auxiliary input at the back of the head.
- This input allows the dividing head to be mechanically linked to the milling machine's leadscrew via a gear train.
- As the milling machine table moves linearly (driven by its leadscrew), the leadscrew's rotation is transmitted through the change gears to the dividing head's worm shaft, which then rotates the workpiece. This synchronized linear and rotary motion creates the helical path. The specific gear ratio in the gear train determines the lead of the helix being cut.
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Spindle and Workholding:
- The spindle is the output shaft of the dividing head, to which various workholding devices like three-jaw chucks, collet chucks, or centers can be mounted. This allows for securely clamping the workpiece during machining operations.
In essence, the universal dividing head combines precise gear reduction with a versatile indexing system and an external synchronization capability for helical operations, making it an indispensable tool in precision machining.
